Conventional free flow evaporators of the falling film type are typically interconnected to provide a number of effects to evaporate a wide variety of liquids. The evaporators are used successfully in many installations, including in the evaporation of black liquor in paper pulp producing facilities.
In a typical free flow evaporator, the heating elements (which are such as shown in U.S. Pat. No. 3,512,239) are disposed in parallel in linear banks within a cylindrical upright vessel. Steam, or other heating vapor, is introduced into each of the heating elements through a header located at the bottom of the vessel, the steam then proceeding upwardly inside each of the heating elements and condensing. The condensate flows downwardly in a countercurrent flow, and is removed through an outlet at the bottom of the heating elements below the level of the steam introduction. Boiling liquor, such as black liquor, is introduced at an intermediate location in the vessel, and is recirculated from a pool at the bottom of the vessel to the top of the vessels and flows in a thin film over both sides of each heating element. A portion of the liquor falling in the film over the heating elements is boiled off to produce a vapor, and the boiled vapor moves horizontally in the vessel, then flows upwardly to an entrainment separator, and then passes through a vapor outlet in the top of the vessel.
While conventional free flow evaporators perform their boiling function very efficiently, there are a number of drawbacks associated therewith. For instance, because the heating elements are arranged in a rectangular array, the packing of the elements within the vessel is not particularly efficient. Also, since the heating vapor enters the bottom of each vessel in a multi-effect array, and exits from the top, substantial lengths of duct must be provided leading from the top of each effect to the bottom of the next effect in series. Heat loss occurs in such long ducts. Additionally, water hammer can occur since the steam enters the bottom of the elements and condensate also leaves through the same structure that introduces the heating vapor. Because a common conduit functions as both the heating vapor inlet and a condensate outlet, the height of the inlet cuts (cutout portions of headers, which cutouts form the heating vapor inlets) of the heating elements are larger than necessary merely for heating vapor introduction since they must ensure that the condensate does not flood out the inlet vapor introduction.
According to the present invention an evaporator, and a method of evaporating a liquid, are provided which overcome the drawbacks associated with conventional free flow evaporating techniques (as set forth above), and provide additional advantages. The two most basic aspects of the evaporator according to the present invention are the provision of the heating elements in a radial configuration so that they are collectively concentric with the vessel and each extends radially within the vessel, and the provision of the heating vapor inlet concentrically at the top of the vessel, with the evaporated vapor outlet still remaining at the top, and the condensate drains to the bottom.
By providing a round package of heating elements in the cylindrical (round) vessel, the symmetry of the design is enhanced and allows more heating elements to be placed within a given diameter vessel, therefore allowing the diameter of the evaporator vessel to be smaller. Also, since in the evaporators according to the present invention, the vapor both enters and leaves from the top of the tank less duct work is necessary (and thus less heat loss) to connect adjacent effects together. Also, the particular positioning of the heating vapor inlet and the circular array of heating elements allows the heating elements to be supported on a bottom beam in such a way that they can expand upwardly. Still further, since the steam enters at the top and the condensate leaves at the bottom a water hammer will not occur, and an additional benefit of the vapor inlet being distinct from the condensate outlet is that the height of the inlet cuts of the heating elements may thus be made much smaller, and smaller inlets are less expensive. Finally, the radial arrangement of the heating elements provides a much stronger design of the elements' package than parallel heating elements, and thus allows a simpler--and less expensive--heating vapor inlet header design.
It is the primary object of the present invention to provide an improved free flow evaporator, and method of evaporating a liquid utilizing interconnected evaporator effects. This and other objects of the invention will become clear from an inspection of the detailed description of the invention, and from the appended claims.